Optical materials using plastics are lightweight, hardly breakable, and excellent in dyeability as compared with optical materials made of inorganic materials such as glass. Therefore, plastic materials of various resins are widely used as optical materials for eyeglass lenses, camera lenses, and the like. Recently, due to an increased demand for higher performance and convenience, studies have continued on optical materials having such properties as high transparency, high refractive index, low specific gravity, high heat resistance, and high impact resistance.
Polythiourethane-based compounds are widely used as optical materials by virtue of their excellent optical characteristics and mechanical properties. A polythiourethane-based compound may be prepared by reacting a polythiol compound and an isocyanate compound. The physical properties of the polythiol compound and the isocyanate compound significantly affect the physical properties of the polythiourethane-based compound to be prepared.
Specifically, if a polymerizable composition that comprises a polythiol compound and an isocyanate compound has a low viscosity at the initial stage of polymerization/curing, striae or bubbles may be generated due to the convection of the composition. Thus, an optical material obtained therefrom, for example, a plastic lens, may be defective.
For example, Japanese Laid-open Patent Publication No. Hei 7-252207 discloses a plastic lens obtained from a tetrathiol and a polyiso(thio)cyanate compound. The tetrathiol used in the above patent publication has a problem in that the viscosity during polymerization tends to be excessively increased because it forms a crosslinked structure at a low temperature, resulting in striae.
In order to prevent the generation of striae, bubbles, and the like, a method of using a compound that contains a hydrogen-bondable functional group to increase the viscosity of the composition at the initial stage of polymerization to an appropriate level may be considered. However, if the compound, which contains a hydrogen-bondable functional group, is used in an excessive amount, the rate of increase in the viscosity may be too fast, which may expedite the generation of striae.